Special Precautions/Comments:

Results should be interpreted in relation to serum concentrations. 

Additional Information:

Urine electrolytes are measured to assess renal handling of specific electrolytes. In the kidney, electrolytes are freely filtered at the glomerulus and undergo bulk reabsorption back into the serum at the proximal tubule of the nephron. Serum electrolyte levels are finely tuned at the distal tubule and reabsorption of sodium in exchange for potassium or H+ ions is controlled by the mineralocorticoid aldosterone. Excess electrolytes are not reabsorbed and excreted in the urine. Disturbances in this process can lead to inappropriate renal loss and low serum electrolyte concentrations.

Measurement is usually performed following a serum U&E to investigate the cause of low electrolyte concentrations. Measurement of urine electrolytes is often performed in conjunction with urine osmolality. Usually a random sample is sufficient, but a 24 hour sample can also be useful. Urine electrolyte concentrations are also dependent on dietary intake. 

Sodium – increased urine sodium (>20 mmol/L) in the presence of hyponatraemia indicates renal loss and may be seen in diuretics, osmotic diuresis (e.g. in diabetes) and mineralocorticoid deficiency. Low urine Na (<10 mmol/L) in the presence of hyponatraemia may indicate extra-renal loss (e.g. vomiting/diarrhoea).  

Potassium – increased urine potassium (>20 mmol/L) in the presence of hypokalaemia suggests renal loss and may be seen in diuretics, osmotic diuresis, mineralocorticoid excess and renal tubular acidosis type 1 and 2. Low urine potassium (<10 mmol/L) in the presence of hypokalaemia may indicate extra-renal loss (e.g. vomiting/diarrhoea) or intracellular shifts (e.g. alkalosis). 

Urea and creatinine are measured to assess renal function. They also indicate how concentrated a urine sample is which can aid interpretation of other analytes. Measurement can also be performed to assess sample integrity (e.g. in a urine drugs of abuse screen).